Effect pigments having a sequence of interference layers applied to a transparent substrate are generally known as pearlescent pigments. The transparent substrate can include natural or synthetic mica, glass flakes, or metal oxides with a platelet shape. Mica, which is inexpensive, readily available and easy to cleave into smooth and thin platelets, is commonly used. In addition, pigments based on mica are stable towards chemical or thermal treatment.
Various techniques have been developed to create color/luster effects. One approach to making these pigments is to coat a platelet substrate with a high refractive index metal oxide layer such as TiO2, Fe2O3, and Zr2O3 or with alternating layers of high and low refractivity as described in U.S. Pat. No. 6,599,355, U.S. Pat. No. 6,500,251 and U.S. Pat. No. 6,648,957.
The use of a metal particle layer as a reflective layer also has been disclosed. For example, U.S. Pat. No. 5,116,664 discloses a titanium-mica composite material comprising mica, a first coating composed of titanium dioxide, and a second coating composed of powder particles of at least one metal selected from the group consisting of cobalt, nickel, copper, zinc, tin, gold and silver.
U.S. Pat. No. 6,794,037 is directed to a high chroma effect material comprising a platelet substrate encapsulated with a layer of silver as light reflective layer, then a spacer layer of metal oxide, nitride, fluoride, and finally an iron oxide layer.
U.S. Pat. No. 6,440,208 discloses a color effect material wherein the platelet substrate is coated first with a light reflective layer selected from the group consisting of copper, zinc, an alloy of copper, and an alloy of zinc, then with a second layer of silicon dioxide or magnesium fluoride, and then a third layer that is selectively transparent to light.
U.S. Pat. No. 6,325,847 describes precious metal color effect materials where a platelet substrate is encapsulated with a reflective precious metal layer, a second layer of silicon dioxide or magnesium fluoride and also with a selective transparent third layer.
U.S. Pat. No. 7,226,503 relates to an effect pigment comprising a glass flake with a thickness of ≦1.0 μm coated with one or more layers of metal oxides, metal suboxides, metal oxyhalides and metal fluorides etc.
U.S. Pat. No. 5,308,394 discloses a pigment that includes a light-transparent ceramic scaly substrate, a thin compound layer coated on a surface of the substrate, a rutile layer titanium dioxide layer formed on a surface of the substrate coated with a tin compound, a metal compound layer coated on a surface of the titanium dioxide layer, the metal compound being at least one selected from the group consisting of Bi, Sb, As, Cd, Mn, Pb and Cr, and metallic glossy dots formed on the surfaces in a scattering manner.
U.S. Pat. No. 6,800,125 discloses an oxide metallic color effect material comprising a platelet-shaped substrate encapsulated with a light reflective silver layer, followed by a layer of iron oxide.
U.S. Pat. No. 6,582,764 discloses a hybrid inorganic/organic color effect material that includes a platelet substrate core coated with a first layer which acts as a reflector to light directed thereon. The first layer can include an alloy of copper and zinc, an alloy of aluminum and copper, an alloy of aluminum and zinc, copper or zinc. The material further includes a second organic polymer layer, and a selectively transparent third layer.
U.S. Pat. No. 6,821,333 discloses a color effect material comprising a platelet-shaped substrate encapsulated with a highly reflective layer of metal selected from silver, gold, platinum, palladium, rhodium, ruthenium etc, a spacer layer of metal oxide, nitride, fluoride or carbide or polymer, and an outer layer selected from metals or metal oxides.
U.S. Pat. No. 6,582,764 is directed to a hybrid/organic color effect material that includes a platelet-shaped substrate encapsulated with three layers. The first layer includes either an alloy of copper and zinc, an alloy of aluminum and copper, or an alloy of aluminum and zinc. The second layer is an organic layer and encapsulates the first layer. The third layer is a selectively transparent layer.
All these pearlescent pigments have an interference color effect and a luster effect. However, the hiding power of such pearlescent pigments is so small that an underlayer cannot be sufficiently covered. Thus, the pigments are either transparent or semi-transparent. Moreover, when such pigments are subjected to a colorimetric appraisal using the CIE lab color space system, the pigments do not exhibit a black color effect while maintaining a pealescent luster effect.
CIElab values are measured with a Multi-angle Spectrophotometer at different angles of 15°, 25°, 45°, 75° and 110°. The reported color coordinates (L, a*, b*) are related to lightness (L) and color (a* and b*). The a* is the red/green content and b* is the blue/yellow content. If a pigment has a low L value with a* and b* values close to zero at a certain angle, this means that the pigment is black at that angle. Further, if the same pigment has a very high lightness (L) value at different angles, this means that the pigment has high light travel property.
Blackness (also called Jetness) can be evaluated using a color dependent black value Mc. Mc is the best jetness parameters so far, and correlates well with the human perception of increased jetness. As Mc increases, the jetness of the masstone inceases. Mc is calculated from tristimulus value of illuminating light source (Xn, Yn, Zn) and the reflected light of sample (X, Y, Z), based on the following equations:L=116(Y/Yn)1/3−16a*=500[(X/Xn)1/3−(Y/Yn)1/3]b*=200[(Y/Yn)1/3−(Z/Zn)1/3]Mc=100[log(Xn/X)−log(Zn/Z)+log(Yn/Y)].An Mc value of 150 or higher is considered highly jet.
Pigments with black color especially at flop angle with high light travel exhibiting high jetness have been widely demanded. Although carbon can be blended so as to create a black effect, such an addition decreases the pearlescent luster effect significantly.
Efforts have been made to obtain dark color effects. U.S. Pat. No. 5,753,024 for example discloses grey pigments that include a substrate coated with tin oxide and at least one further metal oxide and further coated with organic colloids that are calcined at temperatures of 900-1100° C. Silver-grey semi-transparent color pigments having mica coated with titania, ferric oxide and tin oxide are known. Black olive semi-transparent pigments that are based on mica and coated with cobalt iron oxide and cobalt oxide are also known. Such pigments have a brown undertone color. However, the above pigments have an undesirable undertone and do not have good hiding power.